Objection 1. It would seem that humility is not a part of modesty or temperance. For humility regards chiefly the reverence whereby one is subject to God, as stated above (Article 3). Now it belongs to a theological virtue to have God for its object. Therefore humility should be reckoned a theological virtue rather than a part of temperance or modesty.
Objection 2. Further, temperance is in the concupiscible, whereas humility would seem to be in the irascible, just as pride which is opposed to it, and whose object is something difficult. Therefore apparently humility is not a part of temperance or modesty.
Objection 3. Further, humility and magnanimity are about the same object, as stated above (1, ad 3). But magnanimity is reckoned a part, not of temperance but of fortitude, as stated above (Question 129, Article 5). Therefore it would seem that humility is not a part of temperance or modesty.
On the contrary, Origen says (Hom. viii super Luc.): "If thou wilt hear the name of this virtue, and what it was called by the philosophers, know that humility which God regards is the same as what they called metriotes, i.e. measure or moderation." Now this evidently pertains to modesty or temperance. Therefore humility is a part of modesty or temperance.
I answer that, As stated above (137, 2, ad 1; 157, 3, ad 2), in assigning parts to a virtue we consider chiefly the likeness that results from the mode of the virtue. Now the mode of temperance, whence it chiefly derives its praise, is the restraint or suppression of the impetuosity of a passion. Hence whatever virtues restrain or suppress, and the actions which moderate the impetuosity of the emotions, are reckoned parts of temperance. Now just as meekness suppresses the movement of anger, so does humility suppress the movement of hope, which is the movement of a spirit aiming at great things. Wherefore, like meekness, humility is accounted a part of temperance. For this reason the Philosopher (Ethic. iv, 3) says that a man who aims at small things in proportion to his mode is not magnanimous but "temperate," and such a man we may call humble. Moreover, for the reason given above (Question 160, Article 2), among the various parts of temperance, the one under which humility is comprised is modesty as understood by Tully (De Invent. Rhet. ii, 54), inasmuch as humility is nothing else than a moderation of spirit: wherefore it is written (1 Peter 3:4): "In the incorruptibility of a quiet and meek spirit."
Reply to Objection 1. The theological virtues, whose object is our last end, which is the first principle in matters of appetite, are the causes of all the other virtues. Hence the fact that humility is caused by reverence for God does not prevent it from being a part of modesty or temperance.
Reply to Objection 2. Parts are assigned to a principal virtue by reason of a sameness, not of subject or matter, but of formal mode, as stated above (137, 2, ad 1; 157, 3, ad 2). Consequently, although humility is in the irascible as its subject, it is assigned as a part of modesty or temperance by reason of its mode.
Reply to Objection 3. Although humility and magnanimity agree as to matter, they differ as to mode, by reason of which magnanimity is reckoned a part of fortitude, and humility a part of temperance.
Source: Summa Theologica (New Advent)